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Mountain beaver decreases brain size over time underground

Penelitian - Modern mountain beaver ancestors have relatively larger brain sizes. Scientists from the University of Toronto Scarborough report findings to Palaeontology that offer rare cases of the brain to be smaller than the size of the body is likely due to lifestyle changes over time.

The mountain beaver (Aplodontia rufa) are rodents that adapt to life in tunnels dug deep underground. But the fossil record shows the 30-million-year-old ancestor better adapting to life in a tree similar to a modern squirrel.

Penelitian Mountain beaver decreases brain size over time underground

"Early squirrels and ancestors of beavers have relatively similar brain sizes," says Ornella Bertrand of the Department of Anthropology at the University of Toronto Scarborough.

The mountain beaver can climb trees like their ancestors though rarely travel too far from burrows and are mostly active at night. Spending life underground and less dependent on the eye has shrunk the area of the neocortex responsible for vision over time.

"The brain is metabolically expensive that requires a lot of energy. So the part of the brain that is not important to survive may have been chosen not to function," Bertrand said.

The researchers compared the virtual endocast where traces the brain made on the inside of the skull and found the part of the brain associated with vision in particular had shrunk over time. As time goes by, the modern mountain otters are less dependent on sight and the neocortex decreases in size.

"There is a relationship between arboreal, neocortical size and vision," Bertrand said.

Modern mountain otters have larger overall brain sizes than their ancestors, but the brain is smaller relative to body size. An evolutionary decline in brain size has been observed in pets including chickens, pigs and dogs, but the beaver is a rare example of a specific lifestyle shift.

Journal : Ornella C. Bertrand et al. Virtual endocasts of fossil Sciuroidea: brain size reduction in the evolution of fossoriality, Palaeontology, 27 June 2018, DOI:10.1111/pala.12378